As the size of electrical components and the corresponding distance between these components on an integrated circuit decreases, all while electrical complexity of the integrated circuit increases, electromagnetic interference becomes a more serious threat to the electrical integrity of these integrated circuits.
In general, electromagnetic interference waves are produced by electrical and magnetic processes, such as the making and breaking of circuits, spark discharges and, also, atmospheric interference, such as thunderstorms. In addition, the high frequency turning on and off of electronic circuits, loose contacts in electrical leads, poor or intermittent metallic contact between metal bonds and components, and electrostatic charges may also produce these undesirable electromagnetic interference waves. In an integrated circuit, radio frequency generating components may cause electromagnetic interference with other nearby circuits on that same integrated circuit or cause electromagnetic interference with other nearby circuits.
The electromagnetic interference waves, usually in the form of pulses, have a wide frequency spectrum. They may enter an electronic circuit by radiation or conduction from a nearby circuit or from other remote electronic units. Likewise, an integrated circuit can emit electromagnetic interference by radiation or conduction which might affect the operation of other electronic circuits. These interference waves may enter directly into an integrated circuit if the integrated circuit is poorly shielded.
One method for decreasing this undesirable electromagnetic interference, which is generated or received, is to shield or screen the affected electrical circuits from the offending components which are generating the interference waves. Shielding or screening is the complete surrounding of the offending electrical system causing the interference or, alternatively, the offended system receiving the interference, by means of a cover made from an appropriate material. The shielding effect of the material is dependent on the conductivity and magnetic permeability of the material, as well as the frequency of the interference waves. Practically all metals are therefore suitable shielding materials. In addition, shields formed from conductive plastics and/or conductive coatings on an appropriate plastic have also been used to shield the effected, non-offending components from the offending components generating the electromagnetic interference.
Good shielding of the non-offending system prevents the interference waves from passing directly into the non-offending integrated circuit or receiver. This is effective; however, it is desirable to prevent any and all propagation of the interference waves. This could be accomplished by eliminating or considerably reducing the interference waves at their point of origin, such as by shielding the offending system, i.e., the radio frequency generating component on an integrated circuit, not the non-offending system. However, at times, it is not possible or practical to shield the offending system; therefore, it may be desirable to shield the offending system or integrated circuit from receiving the radiated interference.
In addition, it is desirable that the electromagnetic interference shield be compatible with manufacturing techniques currently used to form integrated circuits. Many methods are known for forming the electrical interconnections between an integrated circuit and the supporting substrate. Tape automated bonding (TAB) is one commonly known method for forming these such electrical interconnections, wherein a plurality of bonds are formed simultaneously on an integrated circuit using thermocompression gang bonding techniques. In an alternative bonding method, a flexible circuit (FLEX) is used to form the electrical interconnections between the integrated circuit in the substrate, the substrate being an integral part of the flexible circuit itself.
It is, therefore, advantageous to provide an electromagnetic interference shield which eliminates the interference waves at their point of origin, thereby preventing propagation of the interference waves to non-offending electrical systems or, alternatively, provides a shield to an integrated circuit from receiving radiated electromagnetic interference. In addition, it is advantageous that the provided for electromagnetic interference shield be compatible with the tape automated bonding technologies or the flexible circuitry manufacturing technologies.